Chromatography separates components of a mixture based on their differential affinity between a stationary phase and a mobile phase.
Stationary Phase
The fixed medium (paper, silica on a TLC plate). Polar substances interact more strongly with polar stationary phases → travel less far.
Mobile Phase
The solvent that moves through the stationary phase. Components dissolve in and travel with the mobile phase at different rates.
Retention Factor (Rf)
\[R_f = \frac{\text{distance traveled by substance}}{\text{distance traveled by solvent front}}\]
📐 Worked Example – Aspirin Purity
Solvent front travels 6.0 cm. A visible spot travels 3.0 cm.
\(R_f = \frac{3.0}{6.0} = 0.50\)
⚠️ Examiner Traps
- Rf values are dimensionless (no units) and must be between 0 and 1
- Measure from the centre of the spot, not the top or bottom edge
- A single spot → pure substance. Multiple spots → mixture/impurities
- If Rf = 0, the component is insoluble in the mobile phase – try a different solvent